The present invention relates to water treatment, in particular to a process for the removal of dissolved organic carbon from water.
The processes used in water treatment are largely a function of raw water quality. Potable water supplies often contain unacceptably high levels of organic compounds dissolved, dispersed or suspended in raw water. These organic compounds are referred to herein as dissolved organic carbon (DOC). Other terms used to describe DOC include total organic carbon, organic colour, colour and natural organic matter. DOC often includes compounds such as humic and fulvic acids. Humic and fulvic acids are not discrete organic compounds but mixtures of organic compounds formed by the degradation of plant residues.
The removal of DOC from water is necessary in order to provide high quality water suitable for distribution and consumption. A majority of the compounds and materials which constitute DOC are soluble and not readily separable from the water. The DOC present in raw water renders conventional treatment difficult and expensive.
The provision of a safe potable water supply often requires treatment of water to make it aesthetically acceptable. The removal of suspended matter and colour is an important aspect of this treatment. Two approaches are commonly used for the removal of suspended matter and colour. One involves coagulation and the other membrane filtration.
In the process involving coagulation, a coagulant is applied to destabilise suspended matter and colour so that they coalesce and form a floc, which can then be physically removed by methods such as floating, settling, filtration or a combination thereof. Coagulants such as alum (aluminium sulphate), various iron salts and synthetic polymers are commonly used in processes for water treatment. However, many raw water sources have high levels of DOC present, which is the main cause of the colour, and the DOC reacts with the coagulant requiring a higher coagulant dose than would be required for removal of suspended matter alone. The bulk of the floc formed may then be removed by sedimentation or flotation and the water containing the remainder of the floc passed through a filter for final clarification. However, even after such treatment the treated water may contain as much as 30-70% of the initial DOC.
In the membrane filtration process the water is filtered through a membrane system. However, where the water contains high levels of DOC the membranes tend to be fouled by the DOC, thereby reducing the flux across the membrane, reducing the life of the membranes and increasing operating costs. Membrane systems designed to handle water containing high levels of DOC have much higher capital and operating costs than conventional membrane systems used for the production of potable water.
Ion-exchange resins have been used in water treatment processes for the removal of DOC by passing water treated to remove turbidity and other suspended particles through ion-exchange resin packed in columns or the like. Passing untreated water through a packed resin can cause the packed resin to become clogged and ineffective, problems similar to those faced in membrane filtration.
The present invention provides a process for the reduction or elimination of DOC from water using ion-exchange resins which can be conveniently separated from the water prior to subsequent treatment and its distribution and consumption. Accordingly, we provide a process for the removal of dissolved organic carbon from water, which process includes the following steps:
a. adding an ion-exchange resin to water containing dissolved organic carbon;
b. dispersing the resin in the water to enable adsorption of the dissolved organic carbon onto the resin; and
c. separating the resin loaded with the dissolved organic carbon from the water.
The ion-exchange resin is dispersed in the water so as to provide the maximum surface area of resin to adsorb the DOC. Dispersal of the ion-exchange resin may be achieved by any convenient means. Typically the resin may be dispersed by mechanical agitation such as stirrers and the like, mixing pumps immersed in the water or air agitation where a gas is bubbled through the water. Sufficient shear needs to be imparted on the water to achieve dispersal of the resin.
In some small scale operations for the ion exchange resin may be dispersed in a semi-fluidized bed provided pumping costs are not economically unfeasable. The use of a semi-fluidized bed is not only a convenient means for dispersal of the ion exchange resin but provides for the ready separation of the loaded resin from the water once DOC is adsorbed onto the ion exchange resin.
Separating the resin loaded with DOC from the water may be achieved by settling or screening or a combination thereof. Screening of the loaded resin from the water may be achieved by any convenient means. The screens may be selected with consideration for the size of resin particles to be removed from the water. The configuration of the screens may be such that clogging of the screens is reduced.
In a preferred embodiment, the ion-exchange resin may be more dense than the water and tend to settle to the bottom of the tank. This settling facilitates the convenient separation of the loaded resin from the water. Settling may be facilitated by the use of tube settlers and the like. The resin may then be collected by various means including vacuum collection, filtration and the like. It is preferable that the separation and collection means do not cause mechanical wear which may lead to attrition of the resin.
When a continuous fully suspended system is used, the resin may conveniently be separated from treated water by gravity settling. Based on resin characteristics, very effective ( greater than 99% solids removal) gravitational settling is achieved in high-rate settling modules with retention times less than 20 minutes.
In a preferred process for separating the ion-exchange resin from the water the bulk of resin particles settle out in the first quarter of the separating basin length which is devoid of settler modules (xe2x80x9cfree-flowingxe2x80x9d settling). Further removal of resin particles (xe2x80x9cenhancedxe2x80x9d settling) from treated water is performed in the settler compartment filled with modules which may be either, tilted plates or tubular modules. The bottom of the settler is designed for collection of resin particles in cylindrical, conical or pyramidal hoppers from which the resin particles are pumped back to the front of the process. In this preferred process some mixing of the settled resin in the hoppers may be required to keep it in a fluid condition and to ensure uniform resin concentration of resin in the recycle system.
The ion-exchange resins suitable for use in the process of the present invention have cationic functional groups. The cationic functional groups provide suitable sites for the adsorption of the DOC.
It is preferred that the ion-exchange resins have a diameter less than 100 xcexcM, preferably in the range of from 25 xcexcM to 75 xcexcM. This size range provides an ion-exchange resin which can be readily dispersed in the water and one which is suitable for subsequent separation from the water. The size of the resins affects the kinetics of adsorption of DOC and the effectiveness of separation. The optimal size range for a particular application may be readily determined by simple experimentation.
It is preferred that the ion-exchange resin is macroporous. This provides the resins with a substantially large surface area onto which the DOC can be adsorbed.
Water treatment processes involve the movement of water by stirring, pumping and other operations which can deleteriously effect the ion-exchange resin. It is preferred that the resin is manufactured from tough polymers with polystyrene crosslinkage. The resin may be selected to give the optimum balance between toughness and capacity.
In the process of the present invention the amount of ion-exchange resin necessary to remove DOC from water is dependent on a number of factors including the level of DOC initially present in the water to be treated, the nature of the DOC, the desired level of DOC in the treated water, salinity, temperature, pH, the number of cycles of the resin prior to regeneration and the rate at which it is desired to treat the water to remove DOC. Typically, the amount of ion-exchange resin used to remove DOC from water will be in the range from 0.5 to 5 ml of wet resin per liter of raw water, preferably 0.5 to 3 ml. Higher resin concentrations may also be useful in removing DOC. Such higher concentrations allow shorter contact times and more effective DOC removal.
High doses of resin can be used to remove up to 90% of the dissolved organic carbon but the relationship is non linear and it may not be economical under normal conditions to add resin at these high doses. Sufficient resin may be added to remove a percentage of the dissolved organic carbon such that the cost of any subsequent treatment used to meet water quality objectives is minimised. For example, we have found that removal of dissolved organic carbon reduces the amount of coagulant required to achieve acceptable product water quality. It may also significantly reduce the capital and operating costs of membrane filtration processes.
Preferred ion-exchange resins are recyclable and regenerable. Recyclable resins can be used multiple times without regeneration and continue to be effective in adsorbing DOC. Regenerable resins are capable of treatment to remove adsorbed DOC and such regenerated resins can then be re-introduced into the treatment process.
We have found that, depending on the amount of resin being employed in the treatment process, the resin can be effectively recycled at least 10 times prior to regeneration and in fact at least 20 times depending on water quality. Thus, in a continuous process only 10% or less of the loaded resin, even merely 5%, has to be taken for regeneration, the remainder can be recycled back into the treatment process.
We have found that the used (or spent) resin may be readily treated to remove the adsorbed DOC. Accordingly, we provide a process which incorporates the following additional steps for regenerating spent ion-exchange resin:
a. adding the spent resin to brine;
b. dispersing the spent resin in the brine for the desorption of the DOC from the resin; and
c. separating the regenerated resin from the brine.
It will be understood that the term brine means any high concentration salt solution capable of causing the desorption of DOC from the resin. High concentration sodium chloride solutions are particularly useful as brine in the present process.
The spent resin may be dispersed in the brine by any convenient means. We have found agitation by mechanical stirring or gas bubble agitation to be particularly convenient.
Separation can be achieved by allowing the regenerated resin to settle or by simply filtering through a mesh of appropriate porosity. We have found that the brine can be recycled and used to regenerate resin for a number of times before it becomes unsuitable for use in the regeneration process. The spent brine can itself be regenerated by passage through a reverse osmosis membrane to separate the DOC from the brine. The DOC thus produced is a useful source of humic and fulvic acids.
An alternative process for regenerating spent or loaded ion exchange resin which requires much less brine for the regeneration process may be particularly useful in a number of applications. We have found that the spent ion exchange resin may be packed into a column and the passage of a relatively small quantity of brine through it can effectively regenerate the ion exchange resin. Accordingly, we provide a process for regenerating spent ion exchange resin including the following steps:
a. packing the spent resin into a column; and
b. passing brine through the packed column for the desorption of the DOC from the resin.
The regeneration of the spent ion exchange resin according to this process employing a packed column of spent resin enables particularly high rates of desorption of the DOC from the resin. We have found that by using this process the recyclability of the resin prior to subsequent regenerations is substantially improved.
Further, the humic and fulvic acids are present in significantly higher concentrations in the elutants from the column and thus are a more convenient and economic source of humic and fulvic acids.
The process of the present invention for removal of DOC from water is particularly useful in water treatment applications for the production of potable water. However, the process could also successfully be applied to other aqueous streams where DOC removal is required, eg: industrial use applications, hospital facilities, mining applications or food processing. The process may also be applied to the treatment of waste water. A variety of organic materials, such as toxins or other contaminants, may be removed from waste water.
We have found that a class of ion-exchange resins is particularly suited to use in the process of the present invention. Ion-exchange resins incorporating magnetic particles, known as magnetic ion-exchange resins agglomerate, sometimes referred to as xe2x80x9cmagnetic flocculationxe2x80x9d, due to the magnetic attractive forces between them. This property renders them particularly suited for this application as the agglomerated particles are more readily removable from the water. Accordingly, we provide a process for the removal of dissolved organic carbon from water, which process includes the following steps:
a. adding a magnetic ion-exchange resin to water containing dissolved organic carbon;
b. dispersing the resin in the water to enable adsorption of the dissolved organic carbon onto the magnetic ion-exchange resin;
c. agglomerating the magnetic ion-exchange resin loaded with the dissolved organic carbon; and
d. separating the agglomerated magnetic ion-exchange resin loaded with the dissolved organic carbon from the water.
The magnetic ion-exchange resin may be dispersed in the water by any of the means described above. Sufficient shear needs to be imparted on the water to overcome the magnetic forces which cause the magnetic ion-exchange resin to agglomerate.
Agglomeration of magnetic ion-exchange resin loaded with DOC is achieved by removing the shear which causes the resin to disperse. In an unstirred tank, the magnetic particles in the resin cause the resin to agglomerate. The agglomeration may be facilitated by the use of tube settlers and other means known to those skilled in the art.
Typically the wet magnetic ion-exchange resin is more dense than the water and once agglomeration has commenced the resin tends to settle quickly to the bottom of the tank. This settling facilitates the convenient separation of the loaded resin from the water. The resin may then be collected by various means including vacuum collection, filtration, magnetic transport such as belts, pipes, disks and drums, pumps and the like. We have found vacuum collection particularly convenient. It is preferable that the separation and collection means do not cause mechanical wear which may lead to attrition of the resin.
It is preferred that the ion-exchange resins have a diameter less than 100 xcexcM, preferably in the range of from 25 xcexcM to 75 xcexcM. The size of the magnetic ion-exchange resin affects the kinetics of absorption of DOC and the effectiveness of agglomeration and settling. The optimal size range for a particular application may be readily determined by simple experimentation.
The magnetic ion-exchange resin can have a discrete magnetic core or have magnetic particles dispersed throughout the resin. In resins which contain dispersed magnetic particles it is preferred that the magnetic particles are evenly dispersed throughout the resin.
A particularly preferred magnetic ion-exchange resin is described in the copending provisional application number PM8070 now filed as a PCT application designated all states including the United States of America and entitled xe2x80x9cPolymer beads and method for preparation thereofxe2x80x9d which application is in the names of Commonwealth Scientific and Industrial Research Organisation and ICI Australia Operations Pty Ltd.
The spent magnetic ion-exchange resin may be treated to remove the adsorbed DOC. Accordingly, we provide a process for regenerating spent magnetic ion-exchange resin including the following steps:
a. adding the spent magnetic ion-exchange resin to brine;
b. dispersing the spent magnetic ion-exchange resin in the brine for the desorption of the DOC from the magnetic ion-exchange resin;
c. agglomerating the regenerated magnetic ion-exchange resin; and
d. separating the regenerated magnetic ion-exchange resin from the brine.
An alternative process for regenerating spent or loaded magnetic ion-exchange resin which requires much less brine for the regeneration process may be particularly useful in a number of applications. We have found that the spent magnetic ion-exchange resin may be packed into a column and the passage of a small quantity of brine through it can effectively regenerate the magnetic ion exchange resin. Accordingly, we provide a process for regenerating spent magnetic ion exchange resin including the following steps:
a. packing the spent resin into a column; and
b. passing brine through the packed column for the desorption of the DOC from the resin.
The regeneration of the spent magnetic ion exchange resin according to this process employing a packed column of spent magnetic resin enables particularly high rates of desorption of the DOC from the magnetic resin. We have found that by using this process the recyclability of the magnetic resin prior to subsequent regenerations is substantially improved.
Further, the humic and fulvic acids are present in significantly higher concentrations in the elutants from the column and thus are a more convenient and economic source of humic and fulvic acids.
The process for the removal of DOC from water is useful in water treatment applications for the production of potable water. The treated water is generally disinfected prior to distribution. The levels of DOC can be as much as 70% of the initial DOC after treatment with conventional processes. This DOC may react with any applied disinfectant to produce by-products. Chlorine is often the preferred disinfectant due its cost, ease of use and the fact that a chlorine residual can be maintained throughout the distribution system to inactivate any contamination that may be introduced after the primary disinfection. Chlorine, however, may react with DOC to form a range of by-products, the most well known being trihalomethanes (THMs). THMs have been identified as possible carcinogens and together with the other possible by-products are identified as a health risk in water treatment guidelines throughout the world. Not only can the DOC form such by-products but the oxidation of the DOC into smaller more biodegradable organics, particularly by the use of ozone as a disinfectant, provides a ready food source for bacteria and may result in the regrowth of bacteria in water storages or distribution systems.
Accordingly, we provide a process for water treatment, which includes the following steps:
a. adding an ion-exchange resin to water containing dissolved organic carbon;
b. dispersing said resin in the water for the adsorption of the dissolved organic carbon onto the resin;
c. separating the resin loaded with the dissolved organic carbon from the water; and
d. disinfecting the water.
The steps of adding, dispersing and separating the ion-exchange resin may be accomplished by the methods described above. The water may be disinfected by any convenient means. It is particularly preferred that chlorine or chloramines are used to disinfect the water prior to its storage and/or distribution.
The magnetic ion-exchange resins may preferably be used in this process. Accordingly, we provide a process for water treatment, which includes the following steps:
a. adding a magnetic ion-exchange resin to water containing dissolved organic carbon;
b. dispersing said magnetic ion-exchange resin in the water for the adsorption of the dissolved organic carbon onto the magnetic ion-exchange resin;
c. agglomerating the magnetic ion-exchange resin loaded with the dissolved organic carbon;
d. separating the agglomerated magnetic ion-exchange resin loaded with the dissolved organic carbon from the water; and
e. disinfecting the water.
The steps of adding, dispersing, agglomerating and separating the magnetic ion exchange resin may be accomplished by the methods described above.
The process of the present invention is readily incorporated into existing water treatment facilities. For example, it may be used in conjunction with membrane filtration to improve the effectiveness of the membranes, increase the flux across membranes and reduce operating costs. For new installations it may either replace membrane filtration, or if membrane filtration is still required, significantly reduce the size and hence capital and operating costs of a membrane filtration plant. In fact, the reduction in capital and operating costs may enable consideration to be given to the installation of membrane filtration rather than coagulation/sedimentation plants thereby substantially reducing the size of the plant and enabling the production of potable water without the addition of chemicals other than for disinfection purposes.
Accordingly, in a further aspect the invention provides a process for the treatment of water which includes the following steps:
a. adding an ion-exchange resin to water containing dissolved organic carbon;
b. dispersing said resin in the water to enable adsorption of the dissolved organic carbon onto the ion-exchange resin;
c. separating the ion-exchange resin loaded with the dissolved organic carbon from the water; and
d. subjecting the water to membrane filtration.
In an alternative process, steps c. and d. above may be combined so that the membrane effects separation of the resin while simultaneously filtering the water.
Many water treatment facilities use a coagulation/sedimentation step in their water purification process. For example, in South Australia a six stage process, which is a typical conventional water treatment process, is used to treat the source water for distribution. The six stages are as follows:
Coagulation/Flocculation;
Sedimentation;
Filtration;
Disinfection;
Storage and Distribution; and
Sludge Dewatering and Disposal.
The process of the present invention may be incorporated into this water treatment process most effectively prior to coagulant addition. Typically, coagulants such as alum (aluminium sulphate), iron salts and synthetic polymers are used. The removal of DOC by the present process results in a substantial reduction in the quantity of coagulant required. In addition the removal of DOC reduces the requirement for subsequent chemical additions and improves the efficiency and/or rate of coagulation, sedimentation and disinfection. This has a beneficial impact on the water quality produced and the size of most facilities required within the water treatment plant including sludge handling facilities. These impacts are particularly convenient in the retrofitting of the process of the present invention as they enable the present process to be conveniently incorporated without substantial change in the overall size of the water treatment plant. Accordingly, in a further aspect the invention provides a process for the removal of dissolved organic carbon from water, which process includes the following steps:
a. adding an ion-exchange resin to water containing dissolved organic carbon;
b. dispersing said resin in the water to enable adsorption of the dissolved organic carbon onto the resin;
c. separating the resin loaded with the dissolved organic carbon from the water; and
d. subjecting the water to coagulation/sedimentation.
Utilising the process of the present invention to remove a high proportion of the dissolved organic carbon, reduces the coagulant dose required and may allow the lower volumes of floc produced to be removed from the water directly by filtration, without the need for prior sedimentation.
Some water treatment processes employ activated carbon as a final polishing treatment to alleviate problems with taste and/or odour, to remove disinfection by-products or to remove any other pollutants. The life of the activated carbon is substantially reduced by the presence of DOC in the treated water. Accordingly, a further advantage of our process is that the useful life of activated carbon may be significantly increased. Accordingly, another useful aspect of the present invention includes the further step of subjecting the treated water to activated carbon.
On greenfield sites the use of the process of the present invention will allow significantly smaller footprint water treatment plants to be designed and constructed. The reduction/elimination of DOC from the water using the process of the present invention may be effected in a relatively small volume basin. This is due to the fact reaction and settling rates of the process. This enables the amount of coagulant used in coagulation/sedimentation processes to be reduced which consequently reduces the size of the sedimentation facilities and the size and cost of the water treatment plant. Likewise the size and cost of membrane systems in membrane filtration plants may be reduced which in turn make membrane filtration systems more economically viable when compared with coagulation/sedimentation plants.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.